1. FIELD OF THE INVENTION
The present invention concerns a hyperfrequency band-cut filter and more particularly a filter formed according to the technique of wave guides transmitting a frequency band and absorbing another frequency band whose average frequency is greater than that of the first.
2. DESCRIPTION OF THE PRIOR ART
The problem of absorbing a first frequency band greater than a second frequency band is relatively difficult in the technique of wave guides, for these latter act as high-pass filters. It is therefore compulsory to accept, in a wave guide having a cut-off frequency, both the higher band of frequencies to be absorbed and the lower band of frequencies to be transmitted. It is however necessary in the elements which follow a duplexer to prevent the emitting frequencies from reaching the parametric receiving amplifiers and to absorb them so that the emitting energy does not return to the duplexer. For that purpose, it is a known method to arrange, above a first rectangular guide accepting the two receiving and emitting bands, several rectangular guides, perpendicular to the first, whose length is 0.5.lambda..sub.g (.lambda..sub.g being the wavelength in the guide for the average of the emitting band) and having a transversal dimension such that it be at the cut for the receiving band frequency. These perpendicular guides form transmission elements ended by resistive charges which have the function of absorbing the emitting band. It is known that the first guide is an over-dimensioned guide at the frequencies of the emitting band and that it allows, at these frequencies, the higher rectangular TE.sub.20 mode to pass, whereas the guide is normally fed in the rectangular TE.sub.10 mode at receiving frequencies. Consequently, to cancel the energy at the emitting frequency, it is necessary to couple the guides at the places where the electric and magnetic fields of the rectangular TE.sub.20 mode are maximum, in other words, on either side of the middle of the first rectangular guide. That property has already been used and known filters comprise two rows of guides perpendicular to the first guide and spaced apart from each other. The disadvantage of that solution is the producing of bulky filters, which are not solid from the mechanical point of view, relatively difficult to machine and expensive.
The filter according to the present invention overcomes these disadvantages. Indeed, the latter is formed by a compact assembly which is easy to machine and has a high efficiency characteristic.